A transmission line is typically used for performing long distance transmission of signals. Impedance matching usually needs to be carried out on either of the sending side and the receiving side of a signal, in other words, make the equivalent impedance of the sending side or receiving side equal to the impedance of the transmission line, ZTL, so as to avoid signal reflection. Such impedance matching is carried out on the receiving side more commonly and an ordinary method is to add a terminating resistor, Rt.
FIG. 1 shows a schematic diagram illustrating a circuit for implementing impedance matching on the receiving side of a transmission line. In FIG. 1, ZTL 102 is the impedance of the transmission line. After the impedance matching is performed by a terminating resistor Rt 104 via the transmission line, a signal enters the internal circuit of chip 106 to be handled. In many practical applications (e.g. E1/T1), two transformers 108 are respectively connected on both sides of the transmission line for varying the amplitude of the signal and transforming the impedance. However, this does not substantially affect the principles and basic characteristics of the circuit. Transformer 108 on the receiving side is shown in FIG. 1. An off-chip discrete device with higher precision is typically adopted as Rt 104 in practical applications since signal refection is sensitive to the degree of impedance matching.
This scheme has a disadvantage that the terminating resistor needs to be selected frequently from multiple terminating resistors by using a switch if the circuit is required to be compatible with transmission lines having different impedances. Multiple resistors in the practical product mean the increase of cost while the switch may bring a problem of low reliability and poor operability.